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kjkcolorado

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Where do you position your drogue? Closer to the fin can, or closer to the payload section?

A little background on my thoughts for asking this question:
1) I flew my first dual deploy rocket a little over a year ago and have just 5 dual deploy flights under my belt.
2) For those flights I positioned my drogue closer to the payload section containing the main chute. Reasoning - with the payload higher than the booster under drogue, you don't have to worry about the main getting fouled by the booster as it deploys.
3) A couple days ago I was searching TRF postings to help with a parachute purchase decision and read a thread that got me thinking. The way I understood that thread was that some position their drogue closer to the fin can. Reasoning - at apogee separation, the drogue slows the fin can first as the payload section continues on its path a little longer. This decreases the risk of the fin can catching up to and colliding with the payload section. However, I would be concerned that this puts you at more risk of the fin can fouling the main deployment in that it is above the payload section under drogue.
 

Handeman

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I would agree with your reasoning. I did DD for my L1 and most rockets since. I've always put the drogue close to the payload. I also watch the way the rocket drops and increase or decrease the drogue size depending on the way the rocket falls. I always want the fin can below the payload to prevent the main from fouling. It can still be fouled if the payload is pointing straight down when it deploys and the payload section falls into the main. That is much more rare then the fin can doing it, but it could happen especially if you have a long shock cord because that would allow the main to open before the payload got down to it.

As for the fin can catching up to and hitting the payload at apogee, I'm sure that can happen, but if you have the right size ports and the electronics fire at apogee, I would be more worried about the main fouling later than any detrimental effects from the fin can and payload sections hitting each other. I would expect most cases would result in some damage to the paint only.

of course, YMMV
 

rharshberger

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+1 to what Handeman said, you never want the fin can above the payload for the stated reasons.
 

mpitfield

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For the more common, booster/AV bay/Payload tube/Nosecone design, my preference is to place the drogue on a "third loop", a few feet back from the base of the AV Bay/Payload tube/Nosecone, using either a 20-25' harness.

I also make it a point to use the first few flights about dialing in the recovery system, so I like to dig into the post flight numbers.

One of the things I make a point of doing is reasonably (highly subjective) overbuilding the airframe as well as over-sizing the recovery components in relation to the size of the build, but under-sizing the chutes. I do this because I want as high a decent rate on drogue as possible, and I prefer to target 25'/s decent on main. This results in a relatively abusive, but controlled, landing.

This should look like an inverted V on decent, with the booster well below the payload/main pointing out to one side, and the nosecone pointing out to the other side as well a away from the drogue a few feet above.
 
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cerving

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I've always mounted the drogue close to the AV bay, with a long shock cord to the booster. I used to use nose-blow for the main almost exclusively, lately I've switched to coupler-blow. Assuming your shock cord to the main chute is somewhat shorter than the shock cord going to the payload/NC, the payload/NC is going to be blown clear and there's much less chance that the main will foul. There is a higher chance that the payload/NC shock cord will get tangled in the payload/booster shock cord, but I'll take a higher chance of the main opening properly over a little detangling job later on.
 

MClark

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The booster is likely to be spinning under drogue so you want it lower than the payload so the fins aren't trying to tangle on the harness.
On big heavy rockets the payload smacking the booster can do considerable damage. Having the harness lengths right so they don't hit is easy.

M
 

kjkcolorado

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Thanks. You are all confirming my initial thoughts.

cerving, thanks for the input on separating coupler from payload vs nose cone from payload for main deployment. Hadn't thought about that before. It's going to take a bit for me to visualize that and decide if I want to try it.
 

AlphaHybrids

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I position mine the same as you, but I typically use a a streamer for the drogue. I've seen a main open into a drogue and get all tangled and not open a few times, so going to a streamer can help eliminate this. Also, with a streamer I get less twisting and spinning of the booster.

Edward
 

JimJarvis50

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Seems like everyone is taking the "put it closer to the AV bay" point of view. I think there is a case for the "put it closer to the fins" approach.

Conventional dual deploy designs just happen to fall in a V shape if the drogue is properly sized. The two parts will generally point in opposite directions, or at least away from each other, and if that is the case, then it won't matter where the drogue is positioned since the main will deploy into clean air. In that case, why not arrange for the harness lengths that prevent the fins from running into the AV bay section? Of course, it's possible that I'm the only one that has problems with rocket parts hitting each other during flight.

The problems I've observed occur when the rocket configuration makes it hard to achieve a V shape, regardless of the drogue size that is used. I see this on rockets that have a relatively shorter (lighter) fin section or relatively larger fins. In these cases, and with a drogue that is too small, the fins fly above the drogue and the AV bay section points straight down. And, if the drogue is too large, both parts hang straight down. Then, the deployment problem is the main deploys downward and then fouls with the AV bay section more often than the fins. In this fouling scenario, it also doesn't matter which part of the harness is longer, so again, why not take the opportunity to help keep the parts from colliding?

My solution to this problem is to first try to select a drogue that makes the parts fly correctly. Whether I can do that or not, I also employ an approach that keeps the chute bundled in the deployment bag until the pilot chute pulls the bag above the other parts (i.e., until the harness between the bag and the AV bay section goes taut). This reduces the chances for fouling and delays the deployment of the main until it is above everything else. This has worked well for me, and for perhaps a dozen or so other flights where the technique was used.

Jim
 

cerving

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Or you can just go drogueless. I typically do that for FG rockets 38mm or less, and cardboard rockets 54mm or less. They naturally separate and point in different directions, no fouling.
 

Steve Shannon

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Seems like everyone is taking the "put it closer to the AV bay" point of view. I think there is a case for the "put it closer to the fins" approach.

Conventional dual deploy designs just happen to fall in a V shape if the drogue is properly sized. The two parts will generally point in opposite directions, or at least away from each other, and if that is the case, then it won't matter where the drogue is positioned since the main will deploy into clean air. In that case, why not arrange for the harness lengths that prevent the fins from running into the AV bay section? Of course, it's possible that I'm the only one that has problems with rocket parts hitting each other during flight.

The problems I've observed occur when the rocket configuration makes it hard to achieve a V shape, regardless of the drogue size that is used. I see this on rockets that have a relatively shorter (lighter) fin section or relatively larger fins. In these cases, and with a drogue that is too small, the fins fly above the drogue and the AV bay section points straight down. And, if the drogue is too large, both parts hang straight down. Then, the deployment problem is the main deploys downward and then fouls with the AV bay section more often than the fins. In this fouling scenario, it also doesn't matter which part of the harness is longer, so again, why not take the opportunity to help keep the parts from colliding?

My solution to this problem is to first try to select a drogue that makes the parts fly correctly. Whether I can do that or not, I also employ an approach that keeps the chute bundled in the deployment bag until the pilot chute pulls the bag above the other parts (i.e., until the harness between the bag and the AV bay section goes taut). This reduces the chances for fouling and delays the deployment of the main until it is above everything else. This has worked well for me, and for perhaps a dozen or so other flights where the technique was used.

Jim
Jim,
How do you size your drogue? Do you aim for a target descent rate or something else?
Thanks!
 

JimJarvis50

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Jim,
How do you size your drogue? Do you aim for a target descent rate or something else?
Thanks!
I'm probably the worst person to answer that question because I almost never see my rockets falling on drogue. If I could watch them, I would just empirically adjust the drogue size to get the right "flight". I'm pretty sure that the drogue size that I use - a 24" PML or 18" spherachute - is about right for my 4" rockets, but if you asked me for proof, I wouldn't be able to provide it.

I have never used descent rate as a criteria or ever run a simulation to calculate it. I don't know what I would do with the data, but I'm actually kind of curious to know how the simulation programs deal with the issue of "how will the rocket fall". If they just calculate a descent rate based on the mass of the rocket and the size of the drogue, then I guess I wouldn't be very interested in the results, since I don't think descent rate is the point of it.

I almost always watch other flights and I provide feedback to the flyer (I have big binoculars). Most of the time, they're happy to get feedback because they typically can't or don't watch the configuration that closely or know how the parts should look as they fall. A few years back, Stu Barrett flew a 6" rocket (I think) to around 20K. The thing fell the entire way in the most perfect V I have ever seen - it was really something.

When I was testing my stabilization system in the forward position, I could simply not get the parts to fly correctly (those were lower flights and I could watch how they fell). The AV bay section with the stabilization equipment made that section heavier, and off balance with respect to the fin section, and there was simply no drogue size that would make that section fly in a configuration other than straight down. I had a couple of flights where the main got snagged. That's when I changed my procedure to delay the opening of the main, and I started getting better results with that "off balance" rocket. I use that approach on most all of my flights now.

Jim
 

AlphaHybrids

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Depends, but typically 6" x 60" is one that gets used most often.

Edward
 

Coop

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I put the streamer on the side close to the fin can. Size is dependent on recovery weight. Most often 12 or 15" x 12 or 15'. As small as 6"x 6', as large as 36"x 36'.

Later!

--Coop
 
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